Heat exchanger

ABSTRACT

A heat exchanger is described in which a plurality of heat exchanger tubes are formed into a tube bundle. The tubes are arranged in a plurality of annular coaxial rows, each row comprising a plurality of the tubes. The tubes in each row are wound helically at a common radius with respect to the axis of the annular rows. Ducts are provided for directing a flow of heated fluid over the tubes, and the ends of respective pairs of the tubes are connected at one end of the tube bundle. Fluid to be heated is supplied to the tubes and received from the tubes at the opposite end of the tube bundle from the end at which the pairs of tubes are connected.

0 United States Patent 1191 1111 3,882,933 Kube May 13, 1975 HEAT EXCHANGER Primar ExaminerCharles J. M hre b s D ,c- 1 f. y Y [75] inventor Leonard J an lego a] Assistant Examiner-Theophll W. Streule, Jr. [73] Assignee General Atomic Company, San Attorney, Agent, or Firm-Fitch, Even, Tabin &

Diego, Calif. Luedeka [22] Filed: Oct. 28, 1971 21 A 1 N 193 371 [57] ABSTRACT 1 pp A heat exchanger is described in which a plurality of heat exchanger tubes are formed into a tube bundle. [52] US. Cl 165/163; 122/32 The tubes are arranged in a plurality of annular coax- [51] Int. Cl. F28d 7/10 ial rows, each row comprising a plurality of the tubes. [58] Field of Search 122/32, 34, 156, 163 The tubes in each row are wound helically at a common radius with respect to the axis of the annular [56] References Cited rows. Ducts are provided for directing a flow of UNITED STATES PATENTS heated fluid over the tubes, and the ends of respective 2,888,251 5/1959 Dalin 165 163 x Pairs ofthefubes are conneFted end Ofthe tube 3,071,119 1/1963 Ammon et al. 122/34 bundle Fluld be heated pp t0 the tubes and 3,130,779 4/1964 Huet 165 163 x received r th tubes at the opposite end of the 3,298,358 1/1967 Alden Jr. 122/34 tube bundle from the end at which the pairs of tubes 3,338,301 8/1967 Romanos-.. are connected. 3,379,244 4/1968 3,438,357 4/1969 4 Claims, 4 Drawing Figures PATENTEQ MAY 1 3W5 SHEET 10? 3 INVENTOR.

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sum aor 3- FIG. 3.

INVENTOR.

LEONARD J. KUBE BY ATTORNEYS HEAT EXCHANGER This invention relates to vapor generators such as are used in connection with the production of steam for driving steam turbines. More particularly, the invention relates to a steam generator which is especially suited for use with a gas cooled nuclear reactor in an electrical power generating facility.

Since the advent of nuclear power reactors, substantial steps have been taken toward the efficient and economical production of electrical power from thermal energy derived from these reactors. An important factor in the attainment of this goal is the operation of such reactors at temperatures sufficiently high to enable the direct production of steam at temperatures and pressures suitable for high efficiency operation of steam turbines. In this connection, present day reactor technology has led to the development of high temperature gas cooled reactors which, when employed with a suitable steam turbine system, have the capability of producing electrical power of a quantity and at a cost which meet requirements of the utility industry.

In general, nuclear power plants employing high temperature gas cooled reactors enclose the reactor in a pressure vessel through which a fluid coolant, such as gaseous helium or carbon dioxide, is circulated to withdraw thermal energy liberated by the reactor. Steam for the operation of the turbines is normally obtained by the transfer of heat from the coolant to the fluid of a water/steam system. Conventionally, such heat transfer is accomplished in a steam generator wherein the thermal energy withdrawn from the reactor is utilized to produce superheated steam.

In such a gas cooled reactor/generator system, it is frequently desirable that the gas be caused to make only a single pass through the steam generator before being returned to the reactor. It is therefore important that the greatest possible amount of heat be withdrawn from the gas during the single pass to achieve maximum efficiency. It is also important, however, that there be as little restriction as possible to gas flow in order that work expended in transporting the gas through the system be held to a minimum. Where, for various reasons including structural economy, the steam generator is included in the same pressure bearing containment vessel as the reactor itself, it is also important that the size of the generator be minimized and that the steam generator or sections thereof be readily removable through necessarily restricted openings in the containment vessel. To meet these ends, steam generators often employ tube bundles comprised of a plurality of helically shaped tubes nested closely together.

In such a nuclear reactor system, steam being generated or reheated must, after its temperature is increased by a flow through a particular tube bundle in a vapor generator, often be re-routed through suitable conduits around the bundle back to the same end of the reactor vessel steam generator from which it entered. This is particularly true where the steam generator is contained in a penetration within a reactor vessel. The necessity for routing either lead-ins or lead-outs back to the same end of the tube bundle may result in a loss in efficiency due to heat transfer when routing around the bundle. Also, the space required for the aforesaid routing requires a larger reactor vessel penetration or a consequent reduction in the size of the tube bundle useable in a given reactor vessel penetration.

Accordingly, it is an object of the present invention to provide an improved vapor generator particularly suited for use in a nuclear reactor system.

Another object of the invention is to provide a vapor generator in which tube bundle size within the vapor generator is maximized for a given available space.

It is another object of the invention to provide a vapor generator in which all lead-ins and lead-outs are at the same end of the tube bundle.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawings wherein:

FIG. 1 is a perspective cut-away view of a portion of a nuclear reactor incorporating the vapor generator of the invention;

FIG. 2 is a schematic full section view of the lower portion of the vapor generator of FIG. 1;

FIG. 3 is a sectional view taken along the line 3-3 of FIG. 2; and

FIG. 4 is a schematic view illustrating an alternative embodiment of the invention.

Very generally, the heat exchanger of the invention comprises at least one tube bundle 1 1 having a plurality of heat exchanger tubes 13. The tubes 13 are arranged in a plurality of coaxial annular rows 15, each row comprising a plurality of the tubes with the tubes in each row being wound helically at a common radius with respect to the axis of the annular rows. Duct means 17 and 19 are provided for directing a flow of heated first fluid over the tubes for heating a second fluid in the tubes. Means 21 connect the ends of respective pairs of the tubes at one end of the tube bundle. At the other end of the tube bundle, means 23 supply the second fluid under pressure to one tube in each of the tube pairs and means 25 receive the .second fluid from the other tube in each of the tube pairs.

Referring now more particularly to the drawings, a portion of a nuclear reactor system incorporating the invention is shown. The nuclear reactor system includes a prestressed concrete pressure vessel 27, suitably supported by means not illustrated, within an appurtenant structure, also not illustrated. Prestressing tendons 29 extend axially through the concrete of the pressure vessel 27, which is generally cylindrical in form, and a plurality of annular grooves 31 are formed in the outer surface of the pressure vessel for accommodating circumferential prestressing bands, not illustrated.

The interior of the pressure vessel 27 includes a main chamber 33 in which a reactor core, not illustrated, is supported. The chamber 33 is provided with a liner 35 of suitable metal anchored to the concrete. The core of the reactor may be of any suitable type, but the illustrated reactor system is of the so-called gas-cooled type. Provision is made for circulating a coolant gas, such as helium or carbon dioxide, over the reactor core, not shown, to raise the temperature of the gas. The gas is then circulated over one or more heat exchangers or vapor generators to produce steam or other vapor for operating machinery to generate electricity. Circulating gas is then returned to the core to be heated once again.

In the illustrated reactor system, the main chamber 33 is surrounded by a plurality of circumferentially spaced cylindrical chambers 37, only one of which is illustrated in the drawings. The chambers 37 are cylindrical in shape and extend vertically within the reactor vessel, having their' axes parallel with the axis of the reactor vessel. A vapor generator and a coolant circulator are disposed in each of the chambers 37. The vapor generator in the drawings is shown generally at 39, and a portion of the coolant circulator is shown generally at 41. Coolant gas is conducted from the main chamber 33 to each of the chambers 37 through horizontal ducts 43, one of which is illustrated in the drawings. The coolant is returned to the chamber 33 for recirculation over the reactor core through a horizontal duct 45. Suitable closures, not shown, are provided at the upper ends of the chambers 33 and 37.

The chamber 37 is accessible from the lower end of the pressure vessel 27 through penetrations 47. Each penetration 47 is provided with a metal liner 49 which extends upward and is welded to a metal liner 51 for the chamber 37. Each penetration 47 is closed and sealed by a concrete plug 53 having an outer metal cladding 55. Suitable penetrations are provided in the plug 53 for entry of the various steam pipes, explained below, into the chamber 37 for conducting water and steam to and from the vapor generator 39.

More particularly, feed water for the vapor generator 39 is conducted from a feed water inlet pipe 57 to a feed water header 59. In the header 59, the feed water is divided among a plurality of feed water inlet tubes 61, passing through a penetration in the plug 53 into the chamber 37. The tubes 61 are then routed along the space between the double walled vapor generator duct means 17 to the region just above the duct 43. Each of the tubes 61 becomes a helical coil to form a main steam bundle 63. At the top of the main steam bundle 63, the tubes 61 are sub-headered by appropriate means, not shown, and tube lead-outs 65 from the subheaders and are passed down through a central tube 67 extending axially the full length of the vapor generator 39. In the region within the chamber 37 below the vapor generator 39, the tubes 65 are routed over to a suitable penetration in the lower access plug 53 to be passed therethrough out of the pressure vessel to a header 69. Steam entering the header 69 from the tubes 65 is then collected in the steam outlet pipe 71 and conducted to the turbines, not shown. The tube bundle 63 constitutes an evaporator-economizer and a superheater, and therefore the steam emerging through the steam pipe 71 is superheated steam.

In the illustrated nuclear reactor system, the invention is incorporated in only the reheater section, explained below. Accordingly, further structural details of the main evaporator-economizer and superheater section of the vapor generator will not be given. For purposes of completeness, however, it should be understood that the main steam bundle 63 may be of any suitable mechanical construction. For example, the main steam bundle may be supported by perforated plates, not shown, through which the tubes comprising the main steam bundle are coiled. These plates may be attached to any suitable portion of the vapor generator to provide support for the plates.

As previously mentioned, the tube bundle 11 in the illustrated embodiment comprises the reheater section of the vapor generator 39. The tube bundle is contained within a plenum defined by the duct means 17 and 19. The duct means 17 comprises an outer double cylindrical wall having an annular support rim 73 at its upper end which is provided with an annular mounting nel 79. Above the dome 77, the main bundle 63 is surrounded by a cylinder or shroud 81 supported by the mounting flange 85 on the mounting flange affixed to the liner 51 of the chamber 37.

Hot helium from the reactor core of the reactor system in the chamber 33 is passed through the duct 43 and into the plenum defined between the walls or cylinders 17 and 19. The hot helium then passes downwardly over the reheater tube bundle 11. The lower end of the cylinder 17 is provided with a floor 87, and the lower end of the cylinder 19 terminates a distance above the floor 87. Accordingly, hot helium or other coolant gas, after passing downwardly over the tube bundle 11, enters the annular space between the inner cylinder 67 and the cylinder 19. It then passes upwardly, parallel with the axis of the vapor generator to the region above the main tube bundle 63. The upper end of the cylinder 81 is closed off by a series of annular wall segments 89. The hot helium emerges from the annular plenum between the cylinder 67 and the cylinder 19 and is then forced outwardly and once again downwardly through the tube bundle 63 in the space defined by the cylinder 19 and the outer cylinder 81. The lower edge of the cylinder 81 terminates a distance above the flange 85, with suitable legs 91 extending downwardly to the flange to support the cylinder. Thus, the hot helium gas, after passing over the tube bundle 63, is forced annularly outward through the space between the lower edge of the cylinder 81 and the flange to enter the space between the cylinder 81 and the liner 51 of the chamber 37. In this region, another 180 change in direction results in the gas flow, such that the gas passes vertically upward outside of the cylinder 81.

In order to maintain circulation of the coolant gas, the gas moving to the region above the vapor generator 39 is collected in a plenum defined by a cup-shaped structure 93. The helium circulator 41 communicates with the space defined by the cup-shaped structure 93 and includes turbine blades 95 which are driven by a suitably powered shaft 97 to force the coolant gas through an annular duct 99. The gas leaves the duct 99 in a region, not illustrated, and enters into the upper portion of the chamber 37, the upper portion being indicated at 101. The upper portion of the chamber 37 above the cup-shaped structure 93 communicates with the duct 45, and thus the circulating gas is forced back through the duct to be returned over the reactor core for reheating.

The means which supply and collect the reheat steam to and from the reheater tube bundle 11 are disposed below the tube bundle. The supply means 23 comprise a header supplied by a cold reheat steam inlet pipe 103. Cold reheat steam is divided among a plurality of feeder tubes which then pass through the penetration plug 53 into the region below the vapor generator 39. The tubes 105 are routed over to a suitable location below the tube bundle 11. As previously mentioned, reheat steam first circulates helically upward through tubes in the reheat bundle 11 and then is returned helically downward. Transfer tubes 107 convey the reheat steam, after its downward helical return flow, from locations distributed about the region below the tube bundle 11, to the region above an access penetration through the plug 53. The tubes 107 are then bent downwardly to pass out of the chamber 37 through the plug 53 and into the header or collection means 25. The hot reheat steam entering the header is then conveyed through a reheat steam outlet pipe 109 to be expanded once more in the turbines, not shown.

The particular construction of the reheater tube bundle 11 may be more clearly seen in FIGS. 2 and 3. The tube bundle itself is comprised of a plurality of annular coaxial rows of tubes, each row comprising a plurality of tubes. In each row of tubes, the tubes are wound helically about a common axis with the same radius. This means that when the reheat steam inlet tubes 105 are bent over beneath the vapor generator 39, they each extend to a different position spaced circumferentially around the bottom of each of the various annular rows of tubes. The same is true of the reheat steam outlet tubes 107. Half of the tubes 13 in the tube bundle 11 are used to convey reheat steam helically upward in the tube bundle, and half of the tubes 13 in the tube bundle 11 are used to convey reheat steam downwardly through a helical path in the tube bundle. Thus, the inlet tubes 105 connect to half of the tubes 13, and the outlet tubes 107 connect to the other half of the tubes 13.

In the embodiment illustrated in FIGS. 2 and 3, the up-flow helical tubes and the down-flow helical tubes are positioned in separate ones of the vertical rows 15. Thus, the innermost row of helical tubes in the tube bundle comprises an up-flow set of tubes so that the steam passes helically upward in the tube bundle, and the next adjacent row comprises a down-flow set of tubes. Each of the tubes in the innermost annular row is connected to a respective one of the tubes in the next adjacent annular row, at the top of the tube bundle, by a U-shaped tube segment 21. This is true for the other rows as well, so that each helical tube which carries steam upwardly in the tube bundle is connected through a U-shaped segment 21 to a helical tube which carries the steam helically downward in the tube bundle. To avoid too tight a radius in the U-shaped tube segments 21, the pairs of tubes are not immediately adjacent each other, but are spaced circumferentially relative to each other a slight distance, as viewed in FIG. 3.

By constructing the tube bundle 11 in the manner described, all lead-ins and lead-outs are at the same end of the bundle. This eliminates heat losses as a result of having to transfer vapor around the bundle from one end to the other. Also, the space which would ordinarily be used for such routing may be utilized by an additional row or rows of tubes, increasing the capacity of the tube bundle. Alternatively, the size of the space required for a given tube bundle size is reduced.

Referring to FIG. 4, an alternative embodiment of the invention is illustrated. In FIG. 4, instead of all of the tubes in a single tube row conducting the steam in the same direction, i.e., upwardly or downwardly, alternate tubes in each row conduct the steam upwardly and downwardly. The U-shaped tube segments 21 thus do not cross over between rows as shown in FIG. 3, but rather cross around to connect to the next adjacent tube in the same row. One advantage of the construction of FIG. 4 is that an even number of tube rows 15 is not required, but only an even number of tubes in each row is required. Moreover, somewhat less height is required to make the cross-over connection between up-flow and down-flow tubes than in the case of the embodiment of FIGS. 2 and 3. Cross-over connection between tubes can be accomplished at the same time the particular row of tubes is being constructed, rather than after assembly of two adjacent rows. This reduces manufacturing costs. Because of the intermixing of upflow and down-flow tubes in each row, a more diffused temperature distribution results than is the case when all of the tubes in a given row are either up-flow or down-flow. Finally, the embodiment of FIG. 4 may offer a more desirable layout for combating flow induced vibrations and may result in less of a steam side pressure drop due to less flow resistance.

It may therefore be seen that the invention provides an improved heat exchanger or vapor generator particularly suitable for use in producing steam in a gascooled nuclear reactor. In particular, the heat exchanger of the invention comprises a tube bundle incorporating helical coils and in which all the lead-in and lead-out tubes may connect at the same end of the tube bundle. The design makes it possible to utilize a larger bundle size for a given available space, and fewer heat transfer losses result. Although the invention has been described in connection with the generation of water vapor, it may be applicable to other types of liquid-vapor systems as well.

Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.

What is claimed is:

l. A heat exchanger comprising a plurality of tube bundles, at least one tube bundle comprising a reheater having a plurality of helical heat exchanger tubes, said tubes being arranged in a plurality of coaxial annular rows along a vertical axis with each tube being in a single row, each row comprising a plurality of said tubes with said tubes in each row being wound helically with respect to the axis of said annular rows. duct means for directing a flow of a heated first fluid downwardly over said tubes along the axis of the helix toward the lower end of said tube bundle and then upwardly through the space defined by the inner one of said annular rows, means at the upper end of said tube bundle interconnecting the ends of respective pairs of said tubes, said interconnected ends being uniformly spaced circumferentially around the top of said annular rows, a first tube sheet and a plurality of interconnecting tubes extending from said first tube sheet, each to one tube in a respective one of said tube pairs, and a second tube sheet and a plurality of connecting tubes extending from said second tube sheet, each to the other tube in the respective ones of said tube pairs, said connecting tubes intersecting the respective tubes to which they extend at different positions uniformly spaced circumferentially around the bottom of said annular rows.

2. A heat exchanger according to claim 1 wherein the tubes of each of said pairs are in adjacent rows.

3. A heat exchanger according to claim 1 wherein the tubes of each of said pairs are inthe same row.

4. A heat exchanger according to claim 1 wherein said connecting means each comprise a U-shaped tube segment. 

1. A heat exchanger comprising a plurality of tube bundles, at least one tube bundle comprising a reheater having a plurality of helical heat exchanger tubes, said tubes being arranged in a plurality of coaxial annular rows along a vertical axis with each tube being in a single row, each row comprising a plurality of said tubes with said tubes in each row being wound helically with respect to the axis of said annular rows, duct means for directing a flow of a heated first fluid downwardly over said tubes along the axis of the helix toward the lower end of said tube bundle and then upwardly through the space defined by the inner one of said annular rows, means at the upper end of said tube bundle interconnecting the ends of respective pairs of said tubes, said interconnected ends being uniformly spaced circumferentially around the top of said annular rows, a first tube sheet and a plurality of interconnecting tubes extending from said first tube sheet, each to one tube in a respective one of said tube pairs, and a second tube sheet and a plurality of connecting tubes extending from said second tube sheet, each to the other tube in the respective ones of said tube pairs, said connecting tubes intersecting the respective tubes to which they extend at different positions uniformly spaced circumferentially around the bottom of said annular rows.
 2. A heat exchanger according to claim 1 wherein the tubes of each of said pairs are in adjacent rows.
 3. A heat exchanger according to claim 1 wherein the tubes of each of said pairs are in the same row.
 4. A heat exchanger according to claim 1 wherein said connecting means each comprise a U-shaped tube segment. 